Testing a circuit assembly that contains a piezoelectric switch

ABSTRACT

A method of testing a circuit assembly that includes a piezoelectric switch may include driving a DC current into the piezoelectric switch. The method may further include measuring the time interval it takes to develop a predetermined voltage across the piezoelectric switch and comparing the measured time interval with a first predetermined time interval and a second predetermined time interval. The method may include identifying the circuit assembly as defective when the measured time interval is either less than the first predetermined time interval or more than the second predetermined time interval, and otherwise identifying the circuit assembly is operational.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional patentapplication No. 61/616,057, filed on Mar. 27, 2012, the disclosure ofwhich is incorporated by reference herein.

BACKGROUND

The present disclosure relates to electrical circuit assemblies. Moreparticularly, the present disclosure relates to testing a circuitassembly that contains a piezoelectric switch. Piezoelectricity is theelectric charge that builds up in certain solid materials and may bereleased in response to mechanical stress. Applying mechanical pressureto the piezoelectric material causes it output a voltage pulse that canbe sensed by electronic circuits. Known methods and apparatus fortesting circuits frequently rely on mechanical parts that are subject toincreased wear over time when compared to piezoelectric materials. Theelectronics industry needs a more reliably way to test circuitassemblies.

BRIEF DESCRIPTION

In one embodiment, testing a circuit assembly that includes apiezoelectric switch may include driving a DC current into thepiezoelectric switch. The piezoelectric switch may be or include apiezoelectric crystal. The method may further include measuring the timeinterval it takes to develop a predetermined voltage across thepiezoelectric switch and comparing the measured time interval with afirst predetermined time interval and a second predetermined timeinterval. The method may include identifying the circuit assembly asdefective when the measured time interval is either less than the firstpredetermined time interval or more than the second predetermined timeinterval, and otherwise identifying the circuit assembly is operational.

In another embodiment, a circuit that tests circuit assemblies thatinclude a piezoelectric switch may include a first circuit that appliesa DC current to a piezoelectric switch. The circuit may include a secondcircuit that measures the time interval it takes to develop apredetermined voltage across the piezoelectric switch. The circuit mayfurther include a third circuit that compares the measured time intervalwith a first predetermined time interval and a second predetermined timeinterval and that identifies the circuit assembly as defective when themeasured time interval is either less than the first predetermined timeinterval or more than the second predetermined time interval, andotherwise identifying that the circuit assembly is operational.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is an electrical diagram showing an exemplary embodiment of acircuit that tests circuit assemblies that include a piezoelectricswitch.

DETAILED DESCRIPTION

Those of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons.

FIG. 1 shows an electrical diagram of an exemplary circuit that testscircuit assemblies that include a piezoelectric switch. The componentvalues shown in the exemplary circuit of FIG. 1 are included forillustrative purposes. Persons of ordinary skill in the art will readilyrecognize that other component values may also be used. The optimalcomponent values selected will depend on a number of designconsiderations commonly known by those in the electronics field. In oneembodiment, a circuit 10 may include piezoelectric switch 12. In oneembodiment, the piezoelectric switch 12 may be or include apiezoelectric crystal. The piezoelectric switch 12 may be connectedbetween ground and the inverting input to a comparator 14. Thenon-inverting input to the comparator 14 may be connected to the commonconnection of resistors R1 and R2 at reference numerals 16 and 18 as avoltage divider between potential V+ and ground. High-value resistors R3and R4, at reference numerals 20 and 22, may form a voltage divider tobias the inverting input of the comparator 14. A capacitor C1 atreference numeral 24 may be connected in parallel with resistor R2 togive the circuit some noise immunity.

The output of the circuit 10 may be line 22 at the output of thecomparator 14. Pressing a button that may be mechanically coupled to thepiezoelectric switch 12 may cause the piezoelectric switch 12 to outputa voltage pulse that changes the state of the comparator 14. Thenegative-going output pulse appears at line 20 coupled to the output ofcomparator 14.

In another embodiment, the value of V+ may be 5V. The value of R1 may be7.5 k-ohms. The value of R2 may be 10 k-ohms. The value of R3 may be 1M-ohm. The value of R4 may be 1 M-ohm. The value of C1 may be 0.1 μF.Using these circuit parameters, the inverting input of comparator 14 maybe biased at about 2.5V while the non-inverting input of comparator 14may be biased at about 2.85V. Persons of ordinary skill in the art willimmediately recognize that other component values could be chosen tobias the inputs of comparator 14 at different voltages.

In operation, embodiments of the piezoelectric switch test describedherein may function such that when a DC voltage is applied to thepiezoelectric switch 12, the piezoelectric switch 12 behaves like acapacitor. One pole of a normally-open double-pole single-throw testswitch 26 may couple the V+ voltage to the piezoelectric switch 12through resistor R5 at reference numeral 28 and C2 at reference numeral30. The common connection of the RC network may be coupled to theinverting input of comparator 14. In such embodiments, the RC timeconstant of the circuit may be a function of the resistance of R5 andthe total capacitance of the piezoelectric switch 12 in parallel withthe capacitance C2, which in one embodiment may be about 100 pF. Personsof ordinary skill in the art will readily appreciate that a constantcurrent source may be employed in place of a resistor.

In some embodiments, the other pole of the normally-open double-polesingle-throw test switch 26 may couple V+ to a start input 32 of a timer34. The output of the comparator 14 may be coupled to a stop input 36 ofthe timer 34 via line 38. The test switch 26 may be a mechanical switchor relay, or it may be a solid-state switch element formed from, forexample, a bipolar or field effect transistor. The timer 34 may be acommonly-used timer such as a 555 timer integrated-circuit, or it may beimplemented as a timer function of a microprocessor or microcontroller.In various embodiments in which the test switch 26 is a solid-stateswitch element, the microprocessor or microcontroller may be used tocontrol the test switch 26.

By applying a voltage to the piezoelectric switch 12 and then timing howlong it takes for the applied voltage to reach the trip point of thecomparator 14, one can be determine that the piezoelectric switch 12 iseither functioning and not damaged, or whether the opposite is true. Ifthe rise time is faster than a first threshold value, which in oneembodiment may be based on only R3 and C5, then the capacitance of thepiezoelectric switch 12 did contribute to the RC time constant and thepiezoelectric switch 12 has failed and is either damaged or open. If therise time is slower than a second threshold value, which in oneembodiment may be based on R3 and C_(switch)+C2, then the piezoelectricswitch 12 is likely to be shorted and has failed. A slow rise time couldalso indicate a poor electrical connection somewhere in the circuit, ora bad component other than the piezoelectric switch 12, such as thecomparator 14. In any event, a slow rise time may identify the assemblyas defective. Otherwise, the circuit may be identified as operational.In one embodiment, the first threshold may be less than or equal toabout 1.8 ms, while the second threshold may be greater than or equal toabout 100 ms. In various embodiments in which the piezoelectric switchis or includes a piezoelectric crystal, C_(switch) may be equal toC_(crystal).

While illustrative embodiments have been disclosed herein, personsordinarily skilled in the art will realize that other embodimentsemploying the inventive principles disclosed herein are possible, andsuch embodiments will readily suggest themselves to such skilledpersons. Accordingly, the present invention should only be limitedwithin the spirit of the claims.

What is claimed is:
 1. A method of testing a circuit assembly thatincludes a piezoelectric switch, comprising: driving a DC current intothe piezoelectric switch; measuring the time interval it takes todevelop a predetermined voltage across the piezoelectric switch;comparing the measured time interval with a first predetermined timeinterval and a second predetermined time interval; and identifying thecircuit assembly as defective when the measured time interval is eitherless than the first predetermined time interval or more than the secondpredetermined time interval, and otherwise identifying the circuitassembly as operational.
 2. The method of claim 1, wherein thepiezoelectric switch is a piezoelectric crystal.
 3. The method of claim1, wherein measuring the time interval includes using a timer.
 4. Themethod of claim 3, wherein the timer is implemented as a function of amicroprocessor.
 5. The method of claim 1, wherein the first and secondpredetermined time intervals indicate that the extent to which thecapacitance of the piezoelectric switch contributed to an RC timeconstant of the circuit.
 6. The method of claim 1, wherein the firstpredetermined time interval is less than or equal to about 1.8 ms. 7.The method of claim 1, wherein the second predetermined time interval isgreater than or equal to about 100 ms.
 8. A circuit that tests circuitassemblies that include a piezoelectric switch, comprising: a firstcircuit that applies a DC current to the piezoelectric switch; a secondcircuit that measures the time interval it takes to develop apredetermined voltage across the piezoelectric switch; and a thirdcircuit that compares the measured time interval with a firstpredetermined time interval and a second predetermined time interval andthat identifies the circuit assembly as defective when the measured timeinterval is either less than the first predetermined time interval ormore than the second predetermined time interval, and that otherwiseidentifies the circuit assembly as operational.
 9. The circuit of claim8, wherein a single pole of a normally-open double-pole single-throwtest switch couples the V+ voltage of the circuit to the piezoelectricswitch through a resistor and a capacitor.
 10. The circuit of claim 9,wherein the capacitance of the capacitor is about 100 pF.
 11. Thecircuit of claim 9, wherein the test switch is a solid-state element.12. The circuit of claim 9, wherein the test switch is controlled by amicroprocessor.
 13. The circuit of claim 8, wherein the third circuitincludes a comparator with a V+ value of about 5V, a first resistor witha value of about 7.5 k-ohms, a second resistor with a value of about 10k-ohms, a third resister with a value of about 1 M-ohm, and a fourthresister with a value of about 1 M-ohm.
 14. The circuit of claim 9,wherein the comparator includes an inverting input biased at about 2.5Vand a non-inverting input biased at about 2.85V.
 15. The circuit ofclaim 8, wherein the piezoelectric switch is a piezoelectric crystal.16. The circuit of claim 8, wherein the first predetermined timeinterval is less than or equal to about 1.8 ms.
 17. The circuit of claim8, wherein the second predetermined time interval is greater than orequal to about 100 ms.